Method for injecting a gaseous reacting agent into a bath of molten metal

ABSTRACT

Method for injecting a gaseous reaction agent into a bath of molten metal by the use of a submerged nozzle to achieve the generation of a mass of very fine bubbles throughout a substantial portion of the bath. The bubbles mix and stir the bath to effect chemical homogeneity with a minimum amount of splashing and slopping of the molten metal and persist for a time sufficient to promote substantially complete reaction of the gas phase with the environmental fluid. By selecting a bubble diameter from a consideration of the desired gas flow rate and appropriate Reynolds number, the gas issuing into the molten bath will disperse into small bubbles of essentially constant diameter to maximize the gas-liquid interfacial area. Additionally, by controlling the pressure of the gas forced through the nozzle, the penetration of the fine bubbles into the bath can be controlled to achieve maximum stirring and optimum bubble retention time within the bath without damaging the refractory lining of the vessel containing the bath.

United States Patent [191 Ramachandran et al.

1 1 Feb. 12, 1974 METHOD FOR INJECTING A GASEOUS REACTING AGENT INTO A BATH OF MOLTEN METAL [73] Assignee: Allegheny Ludlum Industries, Inc.,

Pittsburgh, Pa.

[22] Filed: July 9, 1971 [2]] App]. No.: 161,040

[52] US. Cl 75/59, 75/60, 75/61 [51] Int. Cl. C216 7/00, C21c 5/30 [58] Field of Search 75/59, 60, 61, 49

[56] References Cited UNITED STATES PATENTS 3,201,104 8/1965 Berry 75/60 X 2,855,293 10/1958 Savard et al. 75/60 2,871,008 l/l959 Spire 75/59 X 3,574,603 4/1971 Rassen Foss 75/49 X 3,330,645 7/1967 De Moustier et al.. 75/60 3,545,960 12/1970 McClellan et al7 75/61 X 3,573,895 4/1971 Ostberg 75/61 X 3,046,107 7/1962 Nelson et al.... 75/59 3,627,294 12/1971 Hill 75/59 X Primary Examiner-Charles N. Lovell Assistant Examiner-Peter D. Rosenberg Attorney, Agent, or Firm-Vincent G. Gioia [5 7 ABSTRACT Method for injecting a gaseous reaction agent into a bath of molten metal by the use ofa submerged nozzle to achieve the generation of a mass of very fine bubbles throughout a substantial portion of the bath. The bubbles mix and stir the bath to effect chemical homogeneity with a minimum amount of splashing and slopping of the molten metal and persist for a time sufficient to promote substantially complete reaction of the gas phase with the environmental fluid. By selecting a bubble diameter from a consideration of the desired gas flow rate and appropriate Reynolds number, the gas issuing into the molten bath will disperse into small bubbles of essentially constant diameter to maximize the gas-liquid interfacial area. Additionally, by controlling the pressure of the gas forced through the nozzle, the penetration of the fine bubbles into the bath can be controlled to achieve maximum stirring and optimum bubble retention time within the bath without damaging the refractory lining of the vessel containing the bath.

7 Claims, 7 Drawing Figures PAIENTEB EH 1 21914 BASIL U. N. IGWE SUNDARESA/V RAMACHA/VDRAN 8 Ar rorney PAIENIEI] FEB! 21974 7 91 8 1 3 MEAN BUBBLE DIAMETER IN INCHES SUNDARESAN RAMA CHANDRA/V 8 BASIL U. N. IGWE Attorney METHOD FOR INJECTING A GASEOUS REACTING AGENT INTO A BATH OF MOLTEN METAL BACKGROUND OF THE INVENTION As is known, many metallurgical processes require the introduction of a gas onto the surface or within a volume of molten metal for the purpose of reacting impurity dements in the melt with the gas phase to form volatile reaction products. Desiliconization, degassing and decarburization are some of the processes that employ such a technique. Thus, to lower the silicon content of molten iron, oxygen is injected into the bath through a topsubmerged lance which is progressively consumed by mechanical erosion and the high temperature of the liquid. In the basic oxygen process (BOP), the refining oxygen is introduced in the form of a jet issuing from a lance positioned above the surface of the bath. The interaction of a gas jet with the liquid metal results in the oxidation of carbon and other elements capable of chemically reacting with the oxidizing gas. However, blowing oxygen onto the surface of a molten metal bath results in considerable splashing and slopping of liquid in the vessel and undesirable oxidation of useful alloying elements.

Ideally, the oxygen or other reacting gas should be injected into or onto the molten bath so as to minimize splashing and slopping, while facilitating maximized gas-liquid interfacial area to insure complete reaction of the gas with impurities in the melt. At the same time, gases such as oxygen must be injected in such a manner to prevent erosion of the refractory vessel walls.

It has been found that the foregoing desirable characteristics can best be achieved by the use of a submerged nozzle, either one which projects into the side of the vessel or one which extends vertically downwardly and is immersed beneath the upper level of the bath. Furthermore, it is preferable to inject the gas beneath the surface of the melt with the use of a plurality of nozzles. The use of a submerged nozzle, per se, is not new and is shown, for example, in U.S. Pat. No. 2,855,293 which discloses a gas injection device protruding into the body of the molten metal from either the bottom or side walls of the treating vessel. The primary objective of this arrangement is to prolong injector life and integrity by the application of high pressures to the gas, the cooling effect of the high pressure jets serving to insure the integrity of the injecting nozzle. U.S. Pat. No. 3,128,324 describes a method whereby the gases are injected both through a bottom tuyere and a topsubmerged lance for purposes of metal purification. U.S. Pat. No. 3,227,547 discloses various modifications of an injector device that incorporate a system of vanes which serve to stir the melt during metal degassing and also impart a shearing force on the gas stream as it emerges from small orifices positioned at various levels of the submerged portion of a vertical top-submerged lance. The vanes further serve as collision planes for issuing gas bubbles. To insure good bubble dispersion within the bath, it is necessary to rotate either the lance or the metal container or both relative to each other.

The devices and methods described in the foregoing prior art patents, while workable, do not necessarily result in maximum stirring and maximum bubble retention time (i.e., maximum gas-liquid interfacial area) to achieve maximum reaction of the gas with impurities in the melt.

SUMMARY OF THE INVENTION In accordance with the present invention, a method is provided for injecting a reacting gas into a metallic bath of maximized reaction efficiency by injecting the gas into the metal bath as small bubbles and by distributing the generated gas bubbles throughout a substantial portion of the liquid to mix and stir the bath to effect chemical homogeneity. Furthermore, by controlling the trajectory of the injected bubbles within the bath, the residence time of the bubbles can be increased to promote complete reaction of the gas phase with the environmental liquid while at the same time preventing impingement of the bubbles on the refractory lining of the reacting vessel which might otherwise cause erosion of the lining.

Specifically, there is provided a method for injecting a decarburizing or the like gas into a molten metal bath comprising the steps of providing a nozzle having a diameter determined from a known gas flow rate and desired Reynolds number in accordance with the equation:

N 4 W/nd p.

where:

N is the desired Reynolds number,

W is the gas flow rate,

t is the gas viscosity, and

d is the nozzle orifice diameter A nozzle with a diameter determined in accordance with the foregoing equation is disposed such'that it projects into and is immersed in the molten metal bath. The pressure of the gas forced through the nozzle is then controlled from a consideration of the desired horizontal and vertical jet penetrations of bubbles from the nozzle in accordance with the empirically derived equations:

L, 0.527 0.87 P d, Cos 0 and L, 4.16 1.526 P d Sin 0 where:

L, and L, are the vertical and horizontal penetrations, respectively, in inches,

P is the pressure of the gas supplied to the nozzle,

d is the nozzle diameter as determined above, and

6 is the angle of the nozzle with respect to vertical,

as shown in FIG. 4.

The above and other objects and features of the invention will become apparent from the following detailed description taken in connection with the accompanying drawings which form a part of this specification, and in which:

FIG. 1 is a cross-sectional view of a single jet nozzle or orifice inserted through the wall of a vessel containing a metal bath to be purified, and in such a manner that the formed fluid jet emerges within the volume of liquid;

FIG. 2 is a cross-sectional view of a molten metal bath into which two or more gas jets are injected through several single-orifice nozzles distributed around the circumference of the reaction vessel whereby each jet covers a restricted portion of the liquid volume;

FIG. 3 is a cross-sectional view of a molten metal bath supplied by one or more multiple-orifice side injectors whereby the bubble coverage of the bath is determined by the manner of distribution and the angle of inclination of the respective orifices on the injector frame;

FIG. 4 is a cross-sectional view of a molten metal bath supplied with a reaction gas by an orifice located at the end of a vertical gas-supply lance, the axis of the orifice and of the-ensuing jet being inclined at an appropriate angle to the vertical axis of the lance;

FIG. 5 is a cross-sectional view of a molten metal bath showing a lance immersed therein which is equipped with a plurality of orifices inclined at suitable angles to the lance axis whereby the jet from each orifice emerges parallel to the axis of the orifice;

FIG. 6 is a cross-sectional view showing a lance immersed within a molten metal bath and provided with swirling jets comprising a plurality of orifices each 'of which is characterized by an elbow of given included angle; and

FIG. 7 is a 'plot of orifice Reynolds number versus mean bubble diameter for various orifice diameters.

With reference now to the drawings, and particularly to FIG. 1, there is shown a reaction vessel comprising an outer shell 10 of steel or the like having an interior refractory lining 12. The upper end of the reaction vessel 10 is open as at 14 to permit the escape of gases, usually through a hood which covers the opening 14. Contained within the reaction vessel is a molten metal bath 16, such as molten steel to be purified. The refining gas, such as oxygen, is injected into the molten metal bath 16 through a side injector comprising a refractory nozzle device 18 projecting through the side of the reaction vessel near the bottom thereof. When oxygen is thus forced through the nozzle device 18, it will generate bubble within the molten metal bath, the bubble envelope being indicated generally by the reference numeral 20. As will be seen, the present invention provides a means for insuring the generation of turbulent bubbles of essentially equal diameters to insure complete reaction of the gaseous reacting agent, such as oxygen, withimpurities in. the metal bath -l6. Additionally, the invention provides a means for controlling the bubble penetration within the bath. In this respect, if the penetration is too great such that the bubble envelope intersects the refractory lining 12, the oxygen within the bubbles will oxidize and erode the refractory lining.

In FIG. 2, another embodiment of the invention is shown wherein elements corresponding to those of FIG. 1 are identified by like reference numerals. In this case, however, a plurality of nozzle devices 22 is circumferentially spaced around the bottom of the reaction vessel to produce a plurality of bubble penetrations. With this arrangement, the molten metal within the bath 16 will rise at the center and then circulate downwardly along the walls of the reaction vessel to the bottom (along the direction of arrows 17) where it again rises upwardly at the center, thereby insuring a complete mixing of the molten metal bath and reaction of the gaseous bubbles with impurities in the melt.

In FIG. 3, another embodiment of the invention is shown wherein the operation is essentially the same as that of FIG. 2 but wherein each nozzle 24 circumferentially spaced around the bottom of the reaction vessel is provided with a plurality of orifices 26. This provides for a further and more complete mixing of the bubbles with the molten metal bath 16.

The side injectors shownin FIGS. l-3 generate liquid circulation patterns, as shown by the arrows 17 in FIG. 2, which follow closely the trajectory of the bubbles within the liquid body upwardly at the center. The flow of liquid is thereafter downward along the vessel walls where the injector or injectors are located. Consequently, provided that jet penetration extends to at least one-half the vessel diameter, a continuous liquid circulation and mixing is observed to occur. While side injectors such as those shown in FIGS. l-3 can be used in accordance with the invention, they usually result in a large amount of liquid splashing and slopping out of the vessel at high gas flow rates. Furthermore, when a single injector is used as in FIG. 1, or when several injectors are located around the vessel in such a manner that the reaction forces engendered by the jets do not exactly counteract each other, there is generated an undesirable rocking movement of the vessel which at times becomes violent, leading to more splashing and slopping.

Accordingly, while not absolutely necessary, it is desired to utilize a top submerged lance such as that shown in FIGS. 4-6. As shown in FIG. 4, the gas is supplied to the liquid bath 16 through a single orifice 28 at the end ofa vertical lance 30 and inclined at an angle 0 with respect to the vertical axis of the lance. As will be seen, the value of 0 is determined by the size of the bath (in terms of depth and width) which, in turn, determines the vertical and horizontal components of jet penetration. The device illustrated in FIG. 4 effects adequate bubble dispersion, provided the principles of the invention are utilized, as well as a liquid circulation motion that follows closely the bubble trajectory. However, to insure optimum bath mixing, the effective horizontal jet penetration should equal at least a third of the distance from the orifice to the opposite wall of the vessel, and the effective vertical jet penetration should equal at least a third of the bath depth.

The devices illustrated in FIGS. 5 and 6 comprise variations of multiple-jet top submerged injection lances. In FIG. 5, a lance 32 is provided with a plurality of circumferentially spaced jets or nozzles 34 at its bottom. The individual jet characteristics are similar to those of FIG. 4; and the angle of orifice inclination, 0, varies from 0l (i.e., vertically upward) depending upon the appropriate value of 0 as determined in a manner hereinafter described.

The device of FIG. 6 is a modification of that illustrated in FIG. 5, possessing all the superior splash, bubble dispersion, and liquid circulation characteristics of the latter. It comprises a plurality of jets or nozzles 36 projecting outwardly from the side of a submerged lance 38. Each nozzle 36 has a first downwardlyinclined portion 36A lying in a plane aligned with the axis of the lance 38 and a second portion 363 which extends downwardly and backwardly (i.e., skewed with respect to the axis of the lance). In this manner, gas issuing from the nozzles 36 will produce a rotating, swirling action within the bath which effects excellent mixing and stirring without splashing.

As was mentioned above, it is desirable, in order to maximize the gas-metal interfacial area, to inject the gaseous reacting agent into the metal bath so as to produce a large number of relatively small turbulent bubbles. Bubble diameter and turbulence is, in turn, a function of orifice diameter and the orifice flow Reynolds number which is defined as:

N Vd p/p. o l

where:

N Reynolds number, V exit velocity of issuing gas jet, d, diameter of the throat of the orifice or nozzle,

p gas density p. gas viscosity, and

W gas mass flow rate.

The variation in mean bubble diameter as a function of the Reynolds number and orifice diameter is shown in FIG. 7. Laminar flow occurs when the Reynolds number, N is less than 10,000; and turbulence occurs when the Reynolds number exceeds 10,000. Furthermore, it will be noted in the laminar flow region (i.e., a reynolds number below 10,000) the mean bubble diameter is a function of orifice diameter, at least until the Reynolds number reaches 4000. In all cases, the bubble size is a function only of the physical system and gas flow conditions through it, and independent of the properties of the liquid in which the bubbles are formed.

From Equation (1) above and FIG. 7, it can be seen that the mean diameter of generated bubbles can be predictably and conveniently controlled by causing gas flow to occur in such a manner that the orifice flow Reynolds number lies in the turbulent zone (i.e., above 10,000), irrespective of orifice size. Under these conditions, the bubbles will be smaller than 0.18 inch in di ameter. That is, bubble diameter d,, for turbulent gas flow is defined as:

In designing a suitable injection system, it is first necessary to determine the mass gas flow rate and this, in turn will be dependent upon the volume and metallurgical characteristics of the molten metal bath. Once this is determined, the correct orifice diameter can be calculated from Equation (1) above using, as a Reynolds number 10,000 or greater. In the case where multiple orifices are employed, the orifice diameter determined in accordance with Equation (1) above will be the diameter of each 'of the individual orifices.

For most gases of metallurgical importance, density and viscosity variations under identical flow conditions are small. Hence, the Reynolds number depends largely on the velocity of gas flow, orifice diameter, and gas mass flow rate. The orifice diameter is dictated by the desired gas flow rate through it which, in turn, is a function of the driving pressure behind the orifice throat. When the ratio of the absolute pressure immediately ahead of the throat, P and at the exit orifice, P, is less than a critical value given by:

where k is the ratio of the gas specific heat at constant pressure and at constant volume, then the theoretical maximum gas mass flow rate realizable through the orifice is given by:

For most diatomic gases of metallurgical value, the specific heat ratio equals 1.404; and if a gas temperature of 60F is assumed, the above expression in Equation (4) can be transformed in terms of the gas volume flow rate per minute, Q, and orifice throat diameter d in inches, as:

When the pressure ratio is less than critical as defined by Equation (3) above, the gas jet velocity at the orifice throat is sonic and the jet exit velocity is at least sonic, depending upon the specific orifice design employed. Straight orifice and convergent nozzles are not capable of providing jet exit velocities in the supersonic range. Therefore, the Reynolds number of gas flow through orifices of these designs is controlled through the gas flow rate and the orifice diameter. It is upon this basis that the orifice diameter can be calculated in accordance with Equation (1) above. On the other hand, convergent-divergent nozzles can generate supersonic jets at pressure ratios less than the critical value given by Equation (3).

The design of a divergent section of a supersonic nozzle is governed by the relationship:

k-l 1/2 1 1/2 1 e o o l 1; (Lye-1) where:

d is the exit diameter of the nozzle and the other symbols are the same as those defined above. The theoretical velocity attainable at the nozzle exit, with throat velocity being sonic, is given by:

where:

V is the sonic velocity at the temperature and pressure of the gas.

It is apparent, therefore, that the exit velocity V, jet driving pressure P,,, orifice throat and exit diameters, and gas flow rate through the orifice are all amenable to easy control. Consequently, the mean bubble diameter is both predictable and controllable, and the orifice diameter d, can be calculated from the foregoing equations.

As was mentioned above, it is also necessary to determine the penetration of the turbulent bubbles within the bath in order to prevent contact of the bubbles with the refractory walls of the vessel and to insure a circulatory mixing of the bubbles with the melt. it has been determined empirically that in molten metal baths, the vertical penetration, L, and the horizontal penetration, L, are governed by the following equations:

L, 0.527 0.87 P d, Cos 6 L,, 4.16 +1.526 P d Sin where:

0 is the angle of inclination of the nozzle as illustrated in FIG. 4, Y P, is the pressure immediately ahead of the throat of the nozzle, and d is the diameter of the nozzle, or the cumulative diameters of the nozzles in the case of multiple jet nozzle arrangements.

' ln practicing the invention, it is first necessary to determine the gas mass flow rate, this being dependent upon the volume and metallurgical characteristics of the molten metal bath to be treated. From this information, and a selected Reynolds number, usually 10,000 or greater, the nozzle diameter d, can be selected from Equation (1) above in the case of a straight nozzle or from Equation (6) above in the case ofa divergent nozzle. Once the nozzle diameter is determined, and from a consideration of the desired horizontal and vertical penetrations as defined by Equations (8) and (9), the desired pressure P, ahead of the throat can be determinedand can be controlled by a pressure regulator ahead of the nozzle or lance submerged in the metal bath. By controlling the pressure, therefore, the desired penetration can be controlled; and since the nozzle diameter has been selected to insure turbulent bubble conditions with the bubble diameter being essentially constant, a refining or other treating process of the metal bath can be controlled to achieve maximum effectiveness.

It can be seen, therefore, that the invention provides a means for supplying a predetermined quantity of gas at a specified flow rate and driving pressure through one or more orifices submerged underneath the surface of a liquid metal bath to be treated, causing the gas phase to emerge into the body of liquid in the form of a high speed jet. The gas phase, upon encountering the liquid body, breaks up into small-diameter bubbles which traverse the liquid bath in a controlled and predetermined manner. Chemical or other interactions ,occur during the liquid-gas contact period resulting in purification of the liquid species.

Although the invention has been shown in connection with certain specific embodiments, it will be readily apparent to those skilled in the art that various changes in method steps can be made to suit requirements without departing from the spirit and scope of the invention.

We claim as our invention:

1. A method of decarburizing a bath of molten metal within a vessel having a refractory lining, characterized by maximum reaction efficiency and mixing without splashing, comprising the steps of selecting and desired gas mass flow rate determined by the volume and metallurgual characteristies of said molten metal, introducing said gas into said molten metal through an orifice having a diameter determined from said gas mass flow rate and a preselected desired Reynolds number of not less than l0,000 in accordance the equation;

where N is the desired Reynolds number, W is the gas mass ,flow rate, is the gas viscosity, and do is the diameter of each individual orifice,

and passing said gas through said orifice at a preselected desired injection pressure in the orifice adapted to assure that said gas will travel through said molten metal without contacting said refractory lining,

and controlling the formation of bubbles of said gas in said molten metal such that substantially all of said bubble will have a diameter not greater than 0.18 inch, by selecting asuitable orifice diameter and injection pressure.

2. The method of claim 1 wherein the injection pressure of gas in the nozzle is determined and controlled in accordance with the equations:

L, 0.527 0.87 P d, Cos 0 L,, 4.16 1.526 P d Sin 0 where L,, is the vertical penetration of the bubbles, L,, is the horizontal penetration of the bubbles, d is the diameter of the nozzle, and 0 is the angle of inclination of the nozzle with respect to vertical.

3. The method of claim 1 wherein there is a plurality of nozzles.

4. The method of claim 1 wherein said nozzle is projected through a side wall of a vessel containing said molten metal bath.

5. The method of claim 1 wherein said nozzle is disposed on the bottom of a lance which projects into and through the surface of said molten metal bath.

6. The method of claim 1 wherein horizontal penetration, of the gas into the bath is equal to at least onethird of the total distance from said orifice to the opposite refractory lining of said vessel, and vertical penetration of the gas into the bath is equal to at least onethird of the depth of said bath.

7. The method of claim 1 wherein, for a preselected desired gas mass flow rate, preselected desired horizontal and vertical penetrations of said gas through said bath, a preselected desired angle of inclination of said nozzle with respect to vertical, and a preselected Reynolds number not less than 10,000, said orifice diameter is selected and adapted in accordance with the equation:

wherein L,, is vertical penetration of gas bubbles, L, is

horizontal penetration of gas bubbles, d is the cumulative orifice diameter, P is the injection pressure, and

6 is the angle of inclination of the nozzle with respect to vertical. 

2. The method of claim 1 wherein the injection pressure of gas in the nozzle is determined and controlled in accordance with the equations: Lv 0.527 + 0.87 Podo Cos theta Lh 4.16 + 1.526 Podo Sin theta where Lv is the vertical penetration of the bubbles, Lh is the horizontal penetration of the bubbles, do is the diameter of the nozzle, and theta is the angle of inclination of the nozzle with respect to vertical.
 3. The method of claim 1 wherein there is a plurality of nozzles.
 4. The method of claim 1 wherein said nozzle is projected through a side wall of a vessel containing said molten metal bath.
 5. The method of claim 1 wherein said nozzle is disposed on the bottom of a lance which projects into and through the surface of said molten metal bath.
 6. The method of claim 1 wherein horizontal penetration, of the gas into the bath is equal to at least one-third of the total distance from said orifice to the opposite refractory lining of said vessel, and vertical penetration of the gas into the bath is equal to at least one-third of the depth of said bath.
 7. The method of claim 1 wherein, for a preselected desired gas mass flow rate, preselected desired horizontal and vertical penetrations of said gas through said bath, a preselected desired angle of inclination of said nozzle with respect to vertical, and a preselected Reynolds number not less than 10,000, said orifice diameter is selected and adapted in accordance with the equation: NRe 4W/( pi do Mu ) wherein NRe is the Reynolds number, W is the gas mass flow rate and Mu is the gas viscosity, and said injection pressure is selected and adapted in accordance with the equations: Lv 0.527 + 0.87 Podo cos theta Ln 4.16 + 1.526 Podo sin theta wherein Lv is vertical penetration of gas bubbles, Ln is horizontal penetration of gas bubbles, do is the cumulative orifice diameter, Po is the injection pressure, and theta is the angle of inclination of the nozzle with respect to vertical. 